Method for enhancing the solderability of a surface

ABSTRACT

A process for enhancing the solderability of a surface through the use of immersion silver deposits is disclosed. In the preferred embodiment two immersion deposits are utilized in sequence. A composition for immersion silver plating is also disclosed.

This application is a divisional of application Ser. No. 08/621,098,filed Mar. 22, 1996 now U.S. Pat. No. 5,733,599.

FIELD OF THE INVENTION

This invention relates generally to a method of treating a surface whichtreatment enhances the solderability of the surface. The method isparticularly useful in the fabrication and assembly of printed circuitboards.

BACKGROUND OF THE INVENTION

Soldering is generally used for making mechanical, electromechanical, orelectronic connections to a variety of articles. The distinction betweenexpected functions of the joints is important because each applicationhas its own specific requirements for surface preparation. Of the threesoldering applications, making electronic connections is the mostdemanding.

In the manufacture of electronic equipment utilizing printed circuits,connections of electronic components to the printed circuits are made bysoldering of the leads of the components to the through-holes,surrounding pads, lands and other points of connection (collectively,"Areas of Connection"). Typically the connection occurs by wavesoldering techniques.

To facilitate this soldering operation, the printed circuit fabricatoris required to arrange that the through-holes, pads, lands and otherpoints of connection are receptive to the subsequent solderingprocesses. Thus these surfaces must be readily wettable by the solderand permit an integral conductive connection with the leads or surfacesof the electronic components. Because of these needs, printed circuitfabricators have devised various methods of preserving and enhancing thesolderability of surfaces.

One means of arranging good solderability of the surfaces in question isto provide the surfaces with a pre-coating of solder. In printed circuitfabrication, however, this method has several drawbacks. Since it is noteasy to selectively provide these areas with solder, all conductiveareas of the board must be solder plated. This can cause severe problemswith the subsequent application of solder mask.

Another means of arranging good solderability of these surfaces is toplate them with a final finish coating of a precious metal such as gold,palladium or rhodium. U.S. Pat. No. 5,235,139 (Bengston, et. al.), theteachings of which are incorporated herein by reference, proposes amethod for achieving this previous metal final finish. Bengston et. al.propose plating the copper areas to be soldered with electrolessnickel-boron, followed by a precious metal coating such as gold. Seealso U.S. Pat. No. 4,940,181 to Juskey, Jr. et al., the teachings ofwhich are incorporated herein by reference for a similar process whichteaches the plating of electroless copper, followed by electrolyticcopper, followed by nickel followed by gold as a solderable surface.These processes work well but are time consuming and expensive.

Various attempts have been made to selectively apply solder to thenecessary areas only. One such method involves use of organic etchresists over the solder plated areas of connection followed by selectivestripping of tin-lead from the copper traces before application of thesolder mask. See U.S. Pat. No. 4,978,423 to Durnwith et. al. See alsoU.S. Pat. No. 5,160,579 to Larson, the teachings of which areincorporated herein by reference, for other known selective solderprocesses.

Soldering directly to copper surfaces has been difficult andinconsistent. These problems are due mainly to the inability of keepingthe copper surfaces clean and free of oxidation throughout the solderingoperation. Various organic treatments have been developed to preservecopper surfaces in a readily solderable state. For example, see U.S.Pat. No. 5,173,130 (Kinoshita) which teaches the use of certain2-alkylbenzimidazoles as copper pre-fluxes to preserve the solderabilityof the copper surfaces. Treatments such as those taught in Kinoshitahave proven successful but there is still need to improve thereliability of the process.

SUMMARY OF THE INVENTION

The current invention proposes the use of an immersion silver coating asan improved solderability preservative for various surfaces,particularly copper surfaces. Preferred compositions for depositing theimmersion silver coating are also disclosed. In the most preferredembodiment of the invention a second immersion coating of a metal morenoble than silver is deposited over the immersion silver coating. Theprocess proposed is a versatile, low cost method for effectivelypreserving the solderability of surfaces, particularly copper surfacesand areas of connection on printed circuit boards.

DETAILED DESCRIPTION OF THE INVENTION

It has been discovered that immersion silver deposits provide excellentsolderability preservatives, which are particularly useful in thefabrication of printed circuit boards. The solderability achievable witha simple immersion silver deposit in printed circuit applications hasunexpectedly been found to exceed that achievable with prior artnickel-gold plating processes such as described in U.S. Pat. No.5,235,139 and unexpectedly exceeds that achievable with other immersiondeposits. As can be seen in the examples to follow, the processes of thecurrent invention yield surfaces which are very solderable under adverseconditions. In printed circuit applications the surfaces are wirebondable.

Immersion plating is a process which results from a replacement reactionwhereby the surface being plated dissolves into solution and at the sametime the metal being plated deposits from the plating solution onto thesurface. The immersion plating initiates without prior activation of thesurfaces. The metal to be plated is generally more noble than thesurface metal. Thus immersion plating is usually significantly easier tocontrol and significantly more cost effective than electroless plating,which requires sophisticated auto catylitic plating solutions andprocesses for activation of the surfaces prior to plating.

The inventors have found the following immersion silver compositionparticularly suitable to use in the processes of the current invention:

a). a soluble source of silver ions;

b). an acid;

c). an imidazole or imidazole derivative; and

d). optionally, an oxidant.

The soluble source of silver ions can be derived from a variety ofsilver compounds. The inventors have found silver nitrate to be mostpreferable. The concentration of silver in the plating solution canrange from 0.1 to 25 grams per liter, but is most preferably present ina concentration of 0.5 to 2 grams per liter.

Although a variety of acids are suitable for use in this formulation,the inventors have found that methane sulfonic acid is most preferred.The concentration of acid in the plating solution can range from 1 to150 grams per liter but is preferably in the range of 5 to 50 grams perliter.

The inventors have discovered that the inclusion of imidazole orimidazole derivative of the following formula has a significant positiveimpact upon the plate produced by immersion plating solutions,particularly immersion silver plating solutions used in the processes ofthis invention: ##STR1## wherein R₁, R₂, R₃ and R₄ are independentlyselected from the group consisting of substituted or unsubstituted alkylgroups, substituted or unsubstituted aryl groups, hologens, nitro groupsand hydrogen.

The inclusion of an imidazole as described above brightens the plateddeposit and improves the integrity and physical properties of theresultant plated deposit. In addition, the imidazole also extends theuseful life of the immersion plating solution. The inventors have foundthat histidine is a particularly preferred imidazole for the purposes ofthese processes.

The inclusion of imidazoles provides significant advantages in immersionplating solutions in general but is particularly useful and advantageousin immersion silver plating. The inventors have found that immersionsilver deposits resulting from plating baths containing imidazoles arebrighter, smoother and more cohesive than immersion silver depositsplated from baths which do not have imidazoles. In addition theimmersion plating baths with imidazoles have longer effective lives thancomparable baths without imidazoles. These same advantages areachievable by the inclusion of imidazoles in other immersion platingbaths, including copper, palladium, gold, rutheneum and rhodium.

Finally, with respect to the immersion silver compositions useful in thepresent invention, the plating solution may, optionally, advantageouslyalso contain an oxidant. The inventors have found that nitro aromaticcompounds most preferably dinito compounds, such as 3,5dinitrohydroxybenzoic acid are preferred in this regard. Theconcentration of such an oxidant in the solution can range from 0.1 to25 grams per liter, but is preferably from 0.5 to 2 grams per liter.

The immersion silver solution can be used in the processes of thecurrent invention at temperatures ranging from room temperature to 200°F. but is preferably used at from 80 to 120° F. The time for immersionin the plating solution can range from 1 to 30 minutes but is preferablyfrom 3 to 6 minutes.

The immersion silver solution of the current invention is thus used toplate a thin layer of silver onto the surface to be soldered. It isbelieved that the resultant silver coating should be from 1 to 100 microinches thick, preferably from 20 to 60 micro inches thick for effectiveenhancement and preservation of the solderability of the surface.Although this process is effective in soldering many surfaces, it isparticularly useful in soldering copper surfaces, such as Areas ofConnection on printed circuit boards.

In the most preferred embodiment of the current invention, the immersionsilver deposit is then plated upon with second immersion coating of ametal more noble than silver, such as gold, palladium, ruthenium orrhodium. The inventors have found that immersion gold is particularlyuseful in this regard.

It was discovered that while a single layer of immersion coating offerssolderability protection and enhancement, due to the inherent porasityof immersion deposits, a double immersion coating consisting of a firstmetal which is more noble than the base metal followed by a second metalwhich is more noble than the first metal provides unexpectedly betterresults. The first metal may consist of silver (as described herein orotherwise) or another metal which is more noble than the base. In thecase of copper as the base, the first metal can consist of silver, tin,palladium, ruthenium, rhodium, bismuth or the like. The second metalmust be chosen such that it is more noble than the first metal. Anexample of a second metal which is workable with the first metals notedabove would be gold. The importance of the invention however is that theprocess consists of two immersion coatings consisting of a first metalwhich is more noble than the base followed by a second metal which ismore noble than the first metal. The most preferred combination ofimmersion coatings where copper or nickel is the base material isimmersion silver, as taught herein, followed by immersion gold.

Although not wishing to be bound by theory, it is believed that thefirst (silver) immersion coating is inherently porous and therefore mayleave some pathways to the underlying (copper) surface exposed. When thesuggested second immersion coating is applied, these more noble metalions have an access not only to the intermediate metal layer, but alsoto the base layer via the pores. Since the difference between thestandard red-ox potential of the base and the top layer is greater thanthat between the base and intermediate layers, the immersion reactionwill proceed with a much faster rate in the pores (ie. on any exposedbase metal). Therefore the most noble top layer metal will completelyseal the base surface before it builds an immersion deposit of adequatethickness on the intermediate metal layer. It is thus believed that thissynergy provides the optimum solderability.

Although this technique may be utilized advantageously over almost anysurface, it is most useful in the fabrication of printed circuit boards,particularly solder mask over bare copper (SMOBC) boards. Thus, infabricating SMOBC boards, the solder mask is applied to the surfaces ofthe board then exposed and developed to reveal the Areas of Connection.These Areas of Connection are then essentially the only exposed areas ofcopper on the board, with the remainder essentially being covered bysolder mask. These exposed Areas of Connection are thus destined to bepoints of attachment, in most cases by soldering, when the electroniccomponents are later placed on the board later in the fabrication cycle.Therefore, the solderability of these exposed points, generally copper,must be enhanced and preserved.

Thus according to the current invention these areas are then preferablycleaned, using an acid cleaner, and subsequently microetched to preparethe surface for acceptable immersion plating. Following this preferredpreparation, the board is immersed in the immersion silver platingsolution, such that a silver deposit of appropriate thickness isachieved. In the most preferred embodiment the board is then immersed ina second immersion plating solution which plates a metal more noble thanthe silver, such that a top coating of this third metal is achieved at athickness of from 0.5 to 25 micro inches.

The invention is further described for illustrative purposes only in thefollowing examples which are in no way limiting of the invention itself.

EXAMPLE I

Several plated through hole printed circuit boards were fabricatedthrough the following cycle:

1. Drill holes through copper clad laminate.

2. Process boards through standard plated through hole cycle to plateelectroless copper in the holes and on the surface.

3. Apply a plating mask.

4. Electrolytically plate copper to desired thickness in the holes andon the exposed circuitry.

5. Electrolytically plate tin in holes and on exposed circuitry to serveas an etch resist.

6. Strip the plating resist.

7. Etch the exposed copper (ie. copper not plated with tin)

8. Strip the tin.

9. Apply, image and develop a soldermask such that the soldermask coversthe substantially entire board surface except for the Areas ofConnection.

10. Clean and microetch the Areas of Connection.

11. Immersion plate the Areas of Connection with the following silverimmersion plating solution to a thickness of 25 micro inches:

Silver Nitrate--1 gr/liter

Methane Sulfonic Acid (70%)--20 ml/liter

3,5 dinitrohydroxy benzoic acid--1 gr/l

1-Histidine--1 gr/l

Water--to 1 liter

Temperature--100° F.

Time--5 minutes

Note: Fresh water rinses were interposed between each chemicalprocessing step above.

The silver plated in a smooth adherent fashion onto the copper surfaces.The boards were then subjected to accelerated aging by exposing them ina humidity chamber to 100% relative humidity at 93° C. for 8 hours. Theboards were then dried at 130° C. for 5 minutes. The boards were dippedfor 10 seconds in no clean/no residue Interflux IF 2005M flux, allowedto drain for 60 seconds then floated in molten solder at 475° F. for 10seconds. The coverage of solder over the metallic surfaces of the boardswas then evaluated on a percentage basis and found to be approximately98.9%.

EXAMPLE II

Several plated through hole printed circuit boards were fabricatedthrough the same process as in Example I, except that after step 11, thefollowing sequence was followed:

12. The boards were plated with immersion gold using MacDermid ImmersionGold XD-6268 (available from MacDermid, Incorporated, 245 FreightStreet, Waterbury, Conn. 06702). The temperature was 180° F. and thetime was 2 minutes.

The boards were then aged and evaluated in the same way as in Example I.The resultant coverage on a percentage basis was found to beapproximately 99.6%.

EXAMPLE III

Several plated through hole printed circuit boards were fabricatedthrough the same process as in Example I, except that after step 10, thefollowing alternate sequence was followed:

11. The boards were plated in electroless nickel using MacDermid PlanarElectroless Nickel at 170° F. for 5 minutes.

12. The boards were plated in MacDermid Immersion Gold XD-6268 at 180°F. for 5 minutes.

The boards were then aged and evaluated in the same way as in Example I.The resultant coverage on a percentage basis was found to beapproximately 58.50%.

What is claimed is:
 1. A process for enhancing the solderability of asurface, which process comprises:a). immersion plating silver onto thesurface prior to soldering; and b). soldering directly upon the silverplate; wherein the silver is plated from a solution comprising animidazole of the following formula: ##STR2## Wherein R₁, R₂, R₃ and R₄are independently selected from the group consisting of substituted orunsubstituted alkyl groups, substituted or unsubstituted aryl groups,halogens, nitro groups and hydrogen.
 2. A process according to claim 1wherein the surface is selected from the group consisting of copper,nickel, and alloys thereof.
 3. A process according to claim 1 whereinthe surface is an area upon a printed circuit board.
 4. A processaccording to claim 1 wherein the silver is plated to a thickness of 1 to100 micro inches.
 5. A process according to claim 3 wherein the surfaceis an area upon a printed circuit board selected from the groupconsisting of copper, nickel, and alloys thereof.